Study On High-temperature Oxidation Resistance Mechanism Of Aluminum-containing Austenitic Heat Resistance Steel Used In 700? Ultra-supercritical Thermal Power Station Boiler

The problems of energy shortage and environmental pollution have always been concerned in the world.With the increasing demand for energy conservation and emission reduction in china,the development of more efficient,large capacity ultra supercritical thermal power units is imperative,which is a serious challenge for the oxidation resistance and high temperature mechanical properties of heat-resistant steel used in ultra supercritical units.In the present,the heat-resistant steel used in critical components of power plant boiler is stilled traditional Cr-containing steel.But,when the temperature is higher than ⁶00–650°C in harsh environments with water vapor or other aggressive species,the Cr₂O₃-based scales would lose their protectiveness and eventually cause failure of the steels.The new Alumina-Forming Austenitic heat-resistant steel?AFA?with excellent high temperature oxidation resistance,creep resistance and lower cost will be the high temperature candidate materials of the next generation of ultra supercritical units.In our study,three new aluminum-containing austenitic heat-resistant steels were designed.The microstructures and chemical compositions of heat-resistant steels were observed by using optical microscope?OM?and scanning electron microscopy?SEM?with energy-dispersive spectrum?EDS?system.The high-temperature oxidation resistance of the heat-resistant steel has been studied by using oxidation weight increasing method,which was depicted as the oxidation kinetics curves.The surface and cross section of oxidized samples were investigated by morphology observation,XRD diffraction and GDS element depth profile analysis.The composition and structure of the oxide film after oxidation at different temperature were compared and analyzed.The oxidation resistance mechanism of the new heat-resistant steels was revealed.The main conclusions are as follows:?1?The microstructures of three different aluminum-containing austenitic heat-resistant steels after solution treatment at 1260 ? for 2 h are homogeneous and the precipitate phases reduce.The undissolved Nb C phase with dispersion distribution improved the strength of the steels.?2?The oxidation kinetics curves of aluminum-containing steels were fitted to follow the parabolic law at 700 ? and 800 ?.With the increase of aluminum contents,the oxidation rate significantly decreased.After oxidation at 900 ? and 1000 ?,the oxidation rate of aluminum-containing steels was significantly faster than that of 700 ? and 800 ?.At these temperatures,the oxidation rate of the test steels increased with the increase of the aluminum contents.?3?At the service temperature?700??,the surface of traditional Cr-containing austenitic heat-resistant steel mainly forms Cr₂O₃ oxide film.As the increase of aluminum contents,the outer layer oxides of austenitic stainless steel transform from Cr₂O₃ to a composite oxide layer structure comprising Cr and Al.When the aluminum content was 2.5% or 3.5%,the oxide film mainly containing Al was formed in the surface.Since the diffusion of element Al is enhanced and the diffusion of element Cr is inhibited,the oxides enriched in Al dramatically contribute to the improved oxidation resistance of austenitic stainless steels at high temperature.?4?At higher temperatures?such as 900 ? and 1000??,aluminum-containing austenitic heat-resistant steels were observed to form an outer layer of MnCr₂O₄ over the chromia scale.A certain amount of subscale internal alumina and internal aluminum nitride emerged,and the Al N showed needle-like and bulk.The amount and size of AlN particle increased with the increase of the Al contents,which resulted in further deterioration of oxidation resistance.At higher temperature,the diffusion of Mn was accelerated and Mn was easy to combine with Cr₂O₃ to form a large amount of MnCr₂O₄ spinel oxide on the surface.AlN formed underneath the oxide layer deprived the Al element in the base,so that the element Al was depleted in the underlying region,and the formation of the Al₂O₃ film could not be maintained.?5?There is no AlN phase below the oxidation temperature at 800 ?.However,some small needle-like and bulk AlN phases appear between the oxide layer and the substrate at 900 ?.With the temperature reaching 1000 ?,the size and amount of AlN particles further increase.